Copper alloys to be used as brazing filler metals

ABSTRACT

The invention relates to low-nickel copper alloys to be used as brazing filler metals, which alloys also contain phosphor, tin and possibly small amounts of manganese. The alloys are produced by means of rapid solidification. Their advantages are low liquidus temperature and narrow mushy zone. The alloys are mainly used for brazing copper and its alloys.

The present invention relates to low-nickel copper alloys to be used asbrazing filler metal, which alloys also contain phosphorus, tin andpossibly small amounts of manganese. The said alloys are produced byrapid solidification, RS. The advantages of these alloys are their lowliquidus temperature and narrow mushy zone. These alloys are mainly usedfor brazing copper and its alloys.

Soldering is the term for joining metallic or ceramic parts of aworkpiece by means of molten metal, i.e. filler metal, the meltingtemperature whereof is lower than that of the parts to be joined. Themolten filler metal wets the surfaces of the parts to be joined, withoutmelting them. When the working temperature of the filler metal is over450° C., the respective term is brazing, and the filler metal is calledbrazing filler metal. The working temperature of the brazing fillermetal depends on its chemical composition. Pure metals have definedmelting points, but with most metal alloys the transformation from solidto liquid state takes place within a certain mushy zone. The top limitof the solid phase is called solidus, and the bottom limit of the liquidphase is called liquidus. Within the temperature range between thesephase limits, the material is composed of a mixture of solid and liquidphase. In mass production, particularly in machine brazing, the mushyzone of filler metal must be narrow, in order to minimize the timeneeded to solidify a joint. In automatic brazing, the mushy zone of thefiller metal must not surpass 50° C. It is also recommendable that theworking temperature of the brazing filler metal be low, because workingtime is costly. The working temperature of filler metal is the lowesttemperature, in which filler metal melts and wets a workpiece to bejoined and where the filler metal spreads, flows and is bound to thebasic material.

Brazing filler metals can be produced by many different methods. Acommon feature of all RS methods is the high cooling rate of alloy,which is produced with the process, as well as the quenching of the meltinto the final shape, without further reduction. The cooling temperaturegenerally is higher than 10⁵ K/s. Owing to the high cooling rate, theprepared alloy is more homogeneous both in composition and structurethan a conventionally produced alloy. These methods enable theproduction of microcrystalline and even amorphous alloys, if thematerial composition is advantageous for the formation of an amorphousphase, and the cooling rate is high enough. The said methods are calledRS (rapid solidification) methods.

The most commonly used additive in copper-based brazing filler metal isphosphorus, because then a separate flux is not needed in the brazing ofcopper. While the only alloying element employed is phosphorus, themelting temperature of the alloy remains fairly high. For eliminatingthis drawback, among others, there are developed alloys which containfor instance silver and cadmium in addition to phosphorus. Amongcadmium-bearing fillers let us mention for example BAg-1, thecomposition whereof is Ag38Cu21Zn22Cd19 (composition given in atomicpercent), and the mushy zone 605°-650° C. Another silver-bearing brazingfiller alloy is BCuP-5, the composition whereof is Ag9Cu80P11, and themushy zone is 633°-680° C. The brazing suitability of theseconventionally produced filler metals is good, but silver as an alloyingelement makes them expensive, and what is more the cadmium contained inBAg-1 is toxic.

In order to replace silver and cadmium bearing brazing filler metals,there are developed alloys which, in addition to phosphorus, containeither nickel alone, or nickel and tin. The brazing filler metal Cu-Ni-Pand its properties are described, among others, in the U.S. Pat. No.4,253,870. These alloys can be produced at least partly as amorphous bymeans of the RS method, when the alloy composition is within the area5-40 atom percent Ni, 15-20 atom percent P, the balance being Cu andincidental impurities. The essential feature for the brazing fillermetal is that it is at least partly amorphous in structure.

The use of tin as an alloying element lowers the working temperature ofthe filler metal, so that the brazing can be carried out at a lowertemperature than with the Cu-Ni-P brazing filler metal. The EP patentpublication 103,805 relates to a Cu-Ni-Sn-P alloy produced by the rapidsolidification method, the composition of the alloy being within thearea 5-52 atom percent Ni, 2-10 atom percent Sn, 10-15 atom percent P,and the balance being copper. In commercial production there are two ofthese alloys , i.e. Metglas 2002, the composition whereof isCu74Ni10Sn2P14 (in atom percents), and the mushy zone 610°-660° C., andMetglas 2005, the composition whereof is Cu76Ni6Sn5P13, and the mushyzone 592°-655° C. The advantage of these filler metals is that theirmushy zone is roughly equal to that of the brazing filler BAg-1, with ahigh silver content, but they are considerably more economical in price.It has been found out that these brazing filler metals have the lowestworking temperature known so far, when copper is brazed without flux.

According to the present invention there is now developed a Cu-Ni-Sn-Pbrazing filler alloy produced by the RS method, where the nickel contenthas been reduced, and consequently there are achieved lower workingtemperatures, stronger joints and higher impact strength, owing to thereduced volume fraction of the fragile nickel phosphide phase. The Nicontent of the novel brazing filler alloy is within the range 0-5 atompercent, advantageously within the range 2.0-4.8 atom percent, the Sncontent 0-15 atom percent, advantageously 1.5-10.0 atom percent, and theP content 10-20 atom percent, advantageously 11.0-16.5 atom percent.

In the brazing filler alloy of the invention, part of the nickel canalso be replaced by manganese. The alloy can contain 0.5 to 5.4 atompercent manganese. The essential novel features of the invention areapparent from the appended patent claims.

The strip-like product can be manufactured among others by the meltspinning, planar flow casting, melt drag, double roller and melt overflow methods. Best suited for industrial production are the planar flowcasting, melt drag and melt spinning methods, the two former whereof canbe used for producing strips at least 30 cm wide. For instance in theplanar flow casting method, the brazing filler alloy which is firstmelted in a ceramic crucible and then overheated, is sprayed through arectangular slot of a nozzle onto the surface of a rapidly rotatingcooling roll. The melt pool formed in between the roll and the nozzle issolidified into continuous strip. In the melt spinning method the nozzleaperture is a round hole, with a diameter of roughly 1 mm, wherefore themaximum width of the strips to be manufactured is about 5 mm. Thethickness of the strips is 0.01-0.05 mm. A chip-like product is obtainedby means of the melt extraction method, and powder by means of theatomizing method or by grinding the above mentioned strip-like orchip-like products. The fine powder produced by means of the gasatomizing method can be mixed to a suitable binder in order to produce apaste suited for brazing. The product of the present invention can beproduced by means of the above described methods.

The invention is further described by means of an example, where the nowdeveloped brazing filler metal is compared to some prior art productsproduced by means of the RS method. The example shows that the liquidustemperature of the new Cu-Ni-Sn-P alloys was 20°-30° C. lower than theliquidus temperatures of corresponding brazing filler alloys with a highnickel content. The tensile strengths were higher than the tensilestrengths of the brazed joints made by the Metglas alloys.

EXAMPLE

The table below presents first the composition and mushy zone as well asthe ultimate tensile strength (UTS) of a copper-copper joints brazedwith four brazing filler strips of the present invention, and then thecorresponding properties of the earlier mentioned brazing filler stripsMetglas 2002 and Metglas 2005. The brazing filler strips of the presentinvention are produced by the planar flow casting method. The mushy zoneof the composition was defined by means of differential thermal analysisin argon atmosphere, while the heating rate was 5 K/min.

In order to measure the ultimate tensile strength, butt joints of squarecopper rods (K1 E 10×10 mm). The thickness of the filler metal layer was0.04 mm. The pieces to be joined were pressed slightly together and thenbrazed in a tube furnace with argon flow. They were kept in the furnacefor 15 minutes, 5 minutes of which time the pieces were at the brazingtemperature. The brazing temperature was 100° C. higher than theliquidus temperature of the alloy. It is worth noticing that the brazingwith the alloys succeeded at remarkably lower temperatures, too. In thetensile tests all brazed pieces were broken at the joint, wherefore themeasured ultimate tensile strengths (UTS) are tensile strengths of thebrazed joints. The appended results show that the tensile strength ofthe brazed joints made by means of the alloys 1 and 2 of the inventionwere higher than those of the brazed joints made by means of the Metglasalloys. The tensile strength of the joint made by the alloy 3 was higherthan that of the brazed joints made by the Metglas 2002 alloy.

                  TABLE                                                           ______________________________________                                                                                Mushy  UTS                            Alloy                                   zone   [N/                            No.   %      Cu     Ni   Mn   Sn   P    [°C.]                                                                         mm2]                           ______________________________________                                        1     at.    78.8   4.3  --   2.1  14.8 608-629                                                                              170                                  %                                                                             wt.    83.8   4.2  --   4.3   7.7                                             %                                                                       2     at.    76     3.2  --   8.4  12.4 583-610                                                                              176                                  %                                                                             wt.    75.5   2.9  --   15.6 6                                                %                                                                       3     at.    77.5   2.4  2.7  2.2  15.2 599-653                                                                              113                                  %                                                                             wt.    82.9   2.4  2.5  4.3   7.9                                             %                                                                       Met-  at.    72     9.9  --   1.8  16.3 625-643                                                                              103                            glas  %                                                                       2002  wt.    77.9   9.8  --   3.7   8.6                                             %                                                                       Met-  at.    74.9   6.3  --   4.1  14.7 595-635                                                                              163                            glas  %                                                                       2005  wt.    78.4   6    --   8.1   7.5                                             %                                                                       ______________________________________                                    

I claim:
 1. A brazing filler alloy produced by means of the rapidsolidification method and for use in brazing copper and copper alloys,said brazing filler alloy consisting essentially of at least 2.0 to 5.0atom percent Ni, 0-15 atom percent Sn, 10-20 atom percent P and 0.5-5.4atom percent manganese, the balance being copper and incidentalimpurities.
 2. The alloy of claim 1 wherein the amount of Ni is withinthe range of 2.0 to 4.8 atom percent.
 3. The alloy of claim 1 whereinthe amount of Sn is within the range of 1.5 to 10.0 atom percent.
 4. Thealloy of claim 1 wherein the amount of P is within the range of 11.0 to16.5 atom percent.